Evidence of a spectral break in the gamma-ray emission of the disk component of Large Magellanic Cloud: a hadronic origin?

Qing-Wen Tang, Fang-Kun Peng, Ruo-Yu Liu, Pak-Hin Thomas Tam, Xiang-Yu Wang

Published 2017-03-06Version 1

It has been suggested that high-energy gamma-ray emission ($>100{\rm MeV}$) of nearby star-forming and starburst galaxies may be produced predominantly by cosmic rays colliding with the interstellar medium through neutral pion decay. Such pion-decay mechanism predicts a unique spectral signature in the gamma-ray spectrum, characterized by a fast rising spectrum (in $\nu F_\nu$ representation) and a spectral break below a few hundreds of MeV. We here report the evidence of a spectral break around 500~MeV in the disk emission of Large Magellanic Cloud, which is found in the analysis of the gamma-ray data extending down to 60 MeV observed by Fermi Large Area Telescope (LAT). The break is well consistent with the pion-decay model for the gamma-ray emission, although leptonic models, such as the electron bremsstrahlung emission, cannot be ruled out completely.